In recent years, the electrophotographic developing system has been employed in a variety of fields. For example, the system is used not only in the field of copiers but also in the fields of printers as output terminals of computers, color copiers, color printers, and the like. In accordance with the progress in those uses, requirements for image quality have been enhanced.
In order to improve the image quality, there have been provided too numerous proposals to mention wherein the image quality is improved by decreasing the diameter of a toner. However, in the conventional toner having the small diameter, when the only diameter of the toner itself is reduced, adhesive force due to a so-called van der Waals' force increases, On the contrary, it becomes difficult to apply desired charging properties to the toner, and the presence of the weakly charged toner and the excessively charged toner increases. As a result, there have been problems such that during the use of a prolonged period of time, fog is caused on the images and stain is caused in a developing unit or a carrier of two component development and the durability is deteriorated.
Furthermore, on the toner decreased in the diameter there has been a problem on charge rising. In order to solve the problem, it has been studied to improve the charge rising by the increase in the specific surface area of a colored toner itself. By doing so, certainly, the charge rising is improved. However, because the specific surface area is large, ambient conditions at the use depend on water adsorption of fine particles added as an external additive, the pH and further, variations in pH. As a result, there has been a problem such that a large variation in quality is caused by the changes in ambient conditions such as low temperature and low humidity, and high temperature and high humidity.
In addition, when utilizing efficiently the toner having the small diameter, the difference in density between the upper part and the lower part on a copy obtained has been found as one of problems.
This is an occurrence in that a carrier which is transported to a surface of a developing sleeve together with a toner is not removed from the surface of the developing sleeve and is transported again to a developing zone; due to this, the carrier transported again to the developing zone carries no desired amount of the toner and the density of an image on a copy of a document decrease gradually from of the upper side.
The decrease in the diameter of a toner enhances physical adhesive force between the toner and a carrier and as a result, the decrease in developability and deterioration in fluidity are caused. Due to these properties, the problem of the difference in density between the upper part and the lower part is caused remarkably.
In addition, the toner decreased in the diameter is highly adhesive to a photoreceptor and poor cleaning on the photoreceptor and defective images such as the formation of white spots are caused.